This invention relates to a pulverizer for producing powdery or particulate products.
As shown in FIG. 17, a pulverizer of this type has a vertical shell 1 and a hollow screw shaft 2 extending vertically in the shell. The shell 1 is filled with pulverizing medium b such as steel balls. Material a to be pulverized is introduced into the shell 1 from the top end thereof with the screw shaft 2 rotating to pulverize the material by friction between the particles of the material and between the particles of the material and the pulverizing medium b. The powdery product c thus produced is carried out of the shell 1 by an upward flow of carrier fluid d such as air or water passing through the shell 1.
In this type of pulverizer, the means for introducing carrier fluid d into the shell is in the form of an outlet port 3 provided at the bottom of the screw shaft 2. Carrier fluid d may be supplied to the outlet port 3 through the hollow screw shaft 2 as shown in FIG. 17.
Heretofore, the outlet port 3 was either a mere opening formed in the bottom end of the screw shaft 2 as shown in FIG. 16 or a plurality of vertical slits formed in the bottom end of the screw shaft 2 as shown in FIG. 17. In other words, the outlet port was formed in the screw shaft 2.
In this arrangement, since the fluid d reaches only the area near the screw shaft 2, that is, only the central part of the shell 1, an upward current is also formed only in the central part. This causes only the pulverized product c in this area to be discharged, with the product in the outer peripheral part of the shell 1 remaining in the shell for a long time and pulverized too finely. Thus, it was difficult to pulverize the material in the shell uniformly.
Also, since the bottom opening in the screw shaft 2 is liable to get clogged by the pulverizing medium and the material to be pulverized, fluid d has to be fed into the shell 1 with a sufficient force to push them aside. This requires a large amount of power. For example, if the fluid d is air, a fan with a large capacity is required.
Further, if the slits are formed in the screw shaft 2, the number and thus the sectional area thereof cannot be increased so much. Thus, a considerable amount of power is necessary to feed a sufficient amount of fluid.
Also, since the pulverizing medium is always in contact with the bottom end of the screw shaft 2 and the slits, the screw shaft 2 tends to be worn remarkably at the bottom edge thereof or at the area surrounding the slits.
In the pulverizer shown in FIG. 16, the inner diameter of the shell 1, the outer diameter of the screw blades 10 and the revolving speed of the screw shaft 2 are determined taking into consideration the diameter of the pulverizing medium and the inclination of the screw blades. But the screw blades and the liners tend to be worn severely. If the revolving speed is reduced in order to reduce wear of the liners, the efficiency of pulverization will drop.